The present invention relates to a voltage controlled oscillator (VCO), and a startup circuit suitable for the voltage controlled oscillator.
A voltage controlled oscillator is used in a PLL (Phase-Locked Loop) circuit or the like. As the voltage controlled oscillator of the PLL circuit, may be mentioned, for example, a ring oscillator comprised of inverters, a differential ring oscillator, etc. Any of such voltage controlled oscillators changes an amount of current supplied to each of inverter circuits or differential circuits used as components of the ring oscillator in accordance with a control voltage to change a delay time of each inverter, thereby changing an oscillation frequency. Thus, the voltage controlled oscillator is provided with a voltage-current converter circuit for converting a control voltage to a control current in addition to an oscillator.
In the voltage-current converter circuit of the conventional voltage controlled oscillator, the control current reaches approximately zero in a region in which the control voltage is low. Thus, the voltage controlled oscillator had the property that it did not oscillate in such a region or became unstable in oscillation.
And the PLL circuit provided with such a voltage controlled oscillator was accompanied by the problem that when an input clock was interrupted due to some reasons, the PLL circuit attempted to lock onto a signal having a low frequency to thereby reduce the control voltage and eventually led to an output stop.
There was also a fear that when the control voltage was low upon power-on or the like, for example, the PLL circuit could not be locked, so that a clock signal outputted from the PLL circuit became unstable.
Therefore, there is proposed such a semiconductor integrated circuit as shown in a patent document 1 (Japanese Patent Application Laid-Open No. 2002-223149) as a PLL circuit oscillatable even when the input of the reference clock is interrupted or it is placed in the region in which the control voltage is low upon power-on or the like.
In this type of semiconductor integrated circuit, a voltage controlled oscillator has a voltage-current converter circuit which converts a control voltage to a control current corresponding to its voltage value, and an oscillator circuit through which an operating current corresponding to the control current generated by the voltage-current converter circuit is caused to flow and which oscillates at a frequency corresponding to its current value. The voltage-current converter circuit is provided with an auxiliary current adding means which is provided in parallel with a MOS (Metal Oxide Semiconductor) transistor whose gate is inputted with the control voltage and which auxiliarly adds a current to a control current based on the control voltage.
In the patent document 1, the auxiliary current is generated at a circuit in which diode-connected MOS transistors are cascade-connected in two stages. Alternatively, the auxiliary current is generated at a circuit in which a resistor and a diode-connected MOS transistor are cascade-connected. Therefore, the value of the auxiliary current changes depending upon a power supply voltage in terms of a circuit configuration. That is, the auxiliary current increases in proportion to the power supply voltage. Since the auxiliary current is simply determined based on a voltage between the power supply and ground, a problem arises in that it is sensitive even to power noise and ground noise.